[Haskell-cafe] getContents and lazy evaluation

Duncan Coutts duncan.coutts at worc.ox.ac.uk
Fri Sep 1 16:46:08 EDT 2006

On Fri, 2006-09-01 at 16:28 -0400, Robert Dockins wrote:
> On Friday 01 September 2006 15:19, Tamas K Papp wrote:
> > Hi,
> >
> > I am newbie, reading the Gentle Introduction.  Chapter 7
> > (Input/Output) says
> >
> >   Pragmatically, it may seem that getContents must immediately read an
> >   entire file or channel, resulting in poor space and time performance
> >   under certain conditions. However, this is not the case. The key
> >   point is that getContents returns a "lazy" (i.e. non-strict) list of
> >   characters (recall that strings are just lists of characters in
> >   Haskell), whose elements are read "by demand" just like any other
> >   list. An implementation can be expected to implement this
> >   demand-driven behavior by reading one character at a time from the
> >   file as they are required by the computation.
> >
> > So what happens if I do
> >
> > contents <- getContents handle
> > putStr (take 5 contents) -- assume that the implementation
> >        	       		 -- only reads a few chars
> > -- delete the file in some way
> > putStr (take 500 contents) -- but the file is not there now
> >
> > If an IO function is lazy, doesn't that break sequentiality?  Sorry if
> > the question is stupid.
> This is not a stupid question at all, and it highlights the main problem with 
> lazy IO.  The solution is, in essence "don't do that, because Bad Things will 
> happen".  It's pretty unsatisfactory, but there it is.  For this reason, lazy 
> IO is widely regarded as somewhat dangerous (or even as an outright 
> misfeature, by a few).
> If you are going to be doing simple pipe-style IO (ie, read some data 
> sequentially, manipulate it, spit out the output),  lazy IO is very 
> convenient, and it makes putting together quick scripts very easy.  However, 
> if you're doing something more advanced, you'd probably do best to stay away 
> from lazy IO.

Since working on Data.ByteString.Lazy I'm now even more of a pro-lazy-IO
zealot than I was before ;-)

In practise I expect that most programs that deal with file IO strictly
do not handle the file disappearing under them very well either. At best
the probably throw an exception and let something else clean up. The
same can be done with lazy I, though it requires using imprecise
exceptions which some people grumble about. So I would contend that lazy
IO is actually applicable in rather a wider range of circumstances than
you might. :-)

Note also, that with lazy IO we can write really short programs that are
blindingly quick. Lazy IO allows us to save a copy through the Handle

BTW in the above case the "bad thing that will happen" is that contents
will be truncated. As I said, I think it's better to throw an exception,
which is what Data.ByteString.Lazy.hGetContents does.


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